Cardiopulmonary function will be evaluated in the newborn infant born into a high altitude environment (3,100 meters). Ventilatory responses, lung volumes, and effective pulmonary blood flow will be measured at 1-5 days, 1-3 months, and 4 years of age. Ventilatory response to hypoxia will be established in order to determine whether the lack of ventilatory response seen in adults who are long-term residents at 3,100 meters has a congenital basis. Preliminary observations on 20 infants between the age of 48 and 72 hours showed that 8 infants had a reduced ventilatory response to hypoxia. Lung volumes, using two techniques, will be able to determine airway trapping and indirectly measure alveolar stability in these infants. Effective pulmonary blood flow measurements will be compared to measurements of similar gestational and sized infants and will help to evaluate the effects of hypoxia on the pulmonary vascular bed of these infants. These studies will help to evaluate cardiopulmonary adaptation of the newborn to altitude and determine lung and cardiovascular parameters in an infant living in a chronic hypoxic environment.